THE NERVOUS MECHANISM OF RESPIRATION. 379 



that, though the analogy of the cardiac nervous mechanism, in which we can 

 anatomically distinguish between augmentor and inhibitory fibres, justifies 

 us in speaking of augmentor and inhibitory and respiratory fibres as exist- 

 ing in the vagus nerve, we are not as yet able to distinguish them by ana- 

 tomical methods. We may further add that, so exquisitely sensitive is the 

 respiratory centre to these afferent impulses stimuli too slight to produce 

 any appreciable effect when applied to afferent nerves connected with 

 an ordinary centre, such as a spinal reflex centre, may produce marked 

 effects on the respiratory centre. For instance, the feeble electric current 

 which is developed when the cut end of a divided vagus is replaced in the 

 wound, the circuit between the cut end and the longitudinal surface of the 

 nerve being closed through the blood or lymph of the wound, is often suffi- 

 cient to develop inhibitory impulses. Again, when the connection of the 

 respiratory centre with the lungs through the vagus nerves is abolished, not 

 by section of the nerves, but by freezing both nerves at some part of the 

 course of each nerve (an operation which, while completely blocking the 

 passage of impulses along the nerve-fibres, does not itself act as a stimulus), 

 the effect on the respiratory movements is much more in the direction of in- 

 creasing and prolonging the inspiratory act than that of slowing the rhythm. 

 Hence it would appear that what -we have previously described as the result 

 of dividing both vagus nerves is partly due to the blocking of natural im- 

 pulses and partly to the section of the nerves, and possibly to electric cur- 

 rents developed as suggested above, acting as stimuli and thus giving rise to 

 artificial impulses. 



311. The double or alternate respiratory action of the vagus nerves, on 

 which we have dwelt above, may be taken as, in a general way, illustrative 

 of the manner in which other afferent nerves and various parts of the cere- 

 brum are enabled to influence respiration. As we have already said, and, 

 indeed, know from daily experience, of all the apsychical nervous centres 

 the respiratory centre is the one which is most frequently and most deeply 

 affected by nervous impulses from various quarters. Besides the changes 

 brought about by the will (and when we breathe voluntarily we probably 

 make use, to some extent, of the normal nervous machinery of respiration, 

 working through this, rather than sending independent volitional impulses 

 direct to the diaphragm and other respiratory muscles), we find that 

 emotions and painful sensations alter profoundly the character of the 

 respiratory movements. And though these effects may be partly indirect 

 (the emotion modifying the heart-beat or the tonus arteries, and so in- 

 fluencing the flow of blood through the respiratory centre), they are 

 chiefly due to the direct action of nervous impulses reaching that centre 

 from higher parts of the brain. So, also, impulses from almost every sen- 

 tient surface or passing along almost every sensory nerve may modify res- 

 piration in one direction or another. The influence in this way of stimuli 

 applied to the skin is well known to all ; but, perhaps, next to the vagus 

 the nerve most closely connected with the respiratory centre is the fifth 

 nerve, branches of which guard the nasal respiratory channels ; the 

 slightest stimulation of the nostrils at once affects the breathing and most 

 frequently arrests it. The effects of stimuli of various strengths brought 

 to bear on various nerves are very varied. Sometimes the result is an 

 increase of inspiration, and that either by a quickening of the rhythm 

 or by an increase of the individual breaths or by a combination of the 

 two. Sometimes the result is inhibition of inspiration, accompanied or 

 not by an increase of expiration, and sometimes, as when the stimulation 

 causes a cough, the expiratory results may be out of all proportion to the 

 modifications of inspiration. While in the case of some nerves, for instance, 



